The present invention relates generally to hydraulic steering systems and, more specifically, to a system for hydraulically maintaining steered wheels in phase without need for a mechanical tie rod.
To maintain phase of the steered wheels of implements such as self-propelled sprayers, the wheels often are tied together hydraulically rather than mechanically so that expensive and cumbersome tie rods are eliminated. Although the wheels are normally maintained in proper phase, very high pressure loads which develop under certain conditions such as operation in soft soils or on side hills can cause the steering unit to leak so that pressure in the steering lines is lost and steering phase is lost. The steering unit components are typically manufactured with loose tolerances to prevent contaminants in the hydraulic system from prematurely wearing seals and liners. Such tolerances, however, increase the problem of leakage and steering phase loss.
An example of lost steering phase is shown in the schematic representation of a prior art sprayer 10 in FIG. 1 having steerable wheels 12. The wheels 12 can toe in or toe out to disrupt proper steering function and put high loads on the suspension system and on the drivetrain. Similar problems can develop on rear wheels 14 if the sprayer 10 has four wheel steering.
Attempts to correct steering phase problems under high loads have included utilization of pilot operated check valves in the hydraulic steering system to hold steering pressure. However, the check valves introduce excessive chatter into the system and have not provided an acceptable solution to the problems. Circuits with check valves and no relief paths can generate damaging high pressures when obstacles are struck or when certain maneuvers are performed.
It is therefore an object of the invention to provide an improved hydraulic steering system for a vehicle such as an agricultural sprayer which overcomes the aforementioned problems.
It is another object of the invention to provide an improved hydraulic steering system for an off-road vehicle which maintains steered wheel phase, even when the wheels are subjected to high force loads, so that need for mechanical tie rods is eliminated. It is another object to provide such a system which maintains pressure in the steering lines under severe operating conditions so that wheel phase is not lost when a hydraulic tie rod system is utilized.
It is yet another object of the invention to provide an improved hydraulic steering system for an implement such as a sprayer having a counterbalance valve between the steering unit and the steering cylinders which maintains pressure in the steering lines to retain proper phase of the steered wheels even under severe wheel load conditions such as encountered when the implement is operating in soft soils or on side hills. It is still another object to provide such a system wherein damaging high pressures are avoided when obstacles are struck or when high force maneuvers are performed.
It is still a further object of the invention to provide an improved hydraulic steering system for an implement having a valve that can be easily added to new systems or retrofitted on old systems, wherein the valve maintains wheel steer synchronization without need for mechanical linkages.
In accordance with the above objectives, a dual counterbalance valve is added in the hydraulic lines between the steering unit and the steering cylinders on an implement. The counterbalance valve maintains pressure in the steering lines by metering hydraulic fluid to and from the cylinders under pressure so that the steered wheels stay in phase. Since the steering system is constantly kept under pressure, the wheels can only be steered by the steering unit and not by tractive soil forces. Therefore, toe in and toe out problems are eliminated without need for a mechanical tie rod connecting the steered wheels. The valve conveniently is added in-line to the steering cylinders so that current machines can be easily retrofitted.